The invention relates to a screw press for separating liquids from solid-liquid mixtures, especially pulp suspensions, which has a casing provided with liquid passages, especially divided into segments, as well as having a screw rotating inside the casing, a shaft, preferably hollow, and a suspension feed area.
It is known that screw presses with a large diameter are not easy to fill evenly because the pressure in the screw press increases further down in the feed area in accordance with the diameter (height). This means that the pressure on the screen is, for example, approximately 0.14 bar higher at the bottom than at the top. With some pulps to be dewatered, which have a low inlet consistency, the differential pressures applied should not exceed 0.05 bar. At higher pressures, the screens become clogged with pulp and this screen surface is lost for dewatering purposes. Particularly with large screw press diameters, additional pressure is needed in the feed area in order to distribute the pulp to be dewatered in the screw press forwards. With pulps that are difficult to dewater, it is often only possible to use a small percentage (e.g. less than 50%) of the screen surface of screw presses with a large diameter compared with that used in screw presses with a small diameter. A possible solution is known, for example, from DE 19715173. This solution, however, has the disadvantage of heavy fine particles contained in the filtrate (e.g. ash) settling in the tray and thus covering the base of the tray, which means that the submerged screen surface again is not utilised.
The aim of the invention is to eliminate this disadvantage and to provide a screw press that can also be utilised in full, even with a large diameter. This is achieved by a filtrate shell being provided directly at one or several screen baskets and by discharge openings being provided in the lower part of the shell. The filtrate can drain off through these openings into the existing filtrate tray or into a collecting pipe. Thus, clogging of the lower screen surface is largely avoided.
An advantageous configuration of the invention is characterised by the filtrate shell being divided into several zones. With this arrangement it is not only possible to maintain the differential pressure at a constant level, but also to set any desired pressure difference in longitudinal direction inside the screw press.
An advantageous further development of the invention is characterised by the individual zones having filtrate overflows at different heights. In this way, a compression curve setting for the screw press can be corrected easily and the screw press adapted to pulps with different dewatering behaviour.
An advantageous further development of the invention is characterised by the discharge openings being formed as discharge holes and by slide valves being provided at these discharge holes to set the drainage cross-section. With this slide valve the cross-section of the discharge holes for the maximum throughput of the screw press can be set such that the desired liquid level is always obtained in the filtrate tray.
A favourable further development of the invention is characterised by the filtrate tray having filtrate overflows. As a result, the maximum retaining height and thus, the maximum counter-pressure can be set in addition.
A favourable configuration of the invention is characterised by the entire screen basket being surrounded by a filtrate tray in the feed area up to the medium-pressure area, where at least one discharge connection is provided as discharge opening and a throttle valve can be provided for setting the pressure. With this arrangement, the differential pressure can be maintained absolutely constant over the full height in the entire low and medium pressure areas of the screw press.
A favourable configuration of the invention is characterised by the throttle valve being connected to a differential pressure control unit (screen inside/outside). Thus, the inlet pressure can be used to build up a pressure difference in longitudinal direction within the screw press to optimise the compression curve.